1. Field of the Invention
The present invention relates generally to electrical power supplies and more particularly to dual-input, or redundant, automatic-switching power supplies.
2. Background Discussion
Many critical or important electrical and/or electronic equipment or systems require an uninterrupted or substantially uninterrupted supply of electrical operating power at a constant, preestablished voltage level.
A few examples of such equipment or systems are main frame computer systems, hospital operating room and critical care equipment, air traffic control systems, police and fire emergency equipment, telecommunication systems, and national security systems. As may be imagined, the failure of a power supply to deliver a constant, preestablished voltage to such critical equipment or systems can have serious and often dangerous consequences.
It is, therefore, common to provide dual-source electrical power supplies or systems having a primary power (voltage) source input and a secondary power (voltage) source input for providing power (e.g., at 240 volts AC or 120 volts AC or any other voltage according to system requirements) to at least one associated electrical power output. This power output is normally internally connected to provide power from the primary power input.
Dual-input power supplies are, however, ordinarily configured for automatically switching the power output from the primary power input to the secondary power input when the voltage from the primary power input drops below a preestablished dropout level. Thus, the secondary power input provides a back-up for the primary power input.
Such dual-input, automatic-switching power supplies typically also provide for automatically switching the power output from the secondary power back to the primary power when the voltage from the primary power input subsequently rises to a pull-in level above the dropout level, as in the case of a temporary power "brown-out" having been corrected by the primary power source.
The external power source for connecting to the primary power input is normally selected as the most reliable, and usually the least expensive, power source, and may be the local electric power company (through a conventional building outlet). The external power source for connecting to the secondary power input may be an alternative power company (through an alternative building outlet) or an in-house electric generator. The two external power sources are ordinarily both connected to the associated primary and secondary power inputs so that power from one source or the other is always provided to the power supply output, failure of both external power sources being considered highly unlikely.
An important design consideration for any dual-input, auto-switching power supply is the time required for switching the power output from the primary power input to the secondary power input when the voltage from the primary power input drops to its dropout level. Also important is the time required for switching from the secondary power input back to the primary power input voltage after the primary voltage has risen back above the dropout voltage level to a pull-in level. Such switching back is important when, for example, the external power source connected to the primary power input is the local power company and the external power source connected to the secondary power input is provided by an electric generator.
It is generally accepted in the industry that the operation of electrical or electronic equipment of the type that would normally be connected for receiving power from the power output of a dual-input, auto-switching power supply will not be affected if the above-described switching times are no greater than about twenty milliseconds.
Another important design consideration relates to mechanical relays that are typically used to provide the above-described power input switching. In order to reduce the shock hazard on exposed metal pins of a second power input if only one external power source is connected, international standard "EN 60950" requires at least a three millimeter relay moving contact spacing gap, source to source.
However, achieving switching times no greater than twenty milliseconds has previously, to the knowledge of the present inventors, not been possible utilizing mechanical relays having at least a three millimeter moving contact gap spacing, source to source. Typically such relay moving contact gap spacing, source to source, has heretofore had to be less than about one millimeter in order to achieve the at least a twenty millisecond switching time. As an example, although very fast switching times can usually be obtained using solid-state relays, the EN60950 minimum clearance for insulation in primary circuits, and between primary and secondary circuits, is not achieved. Moreover, the failure mode of solid-state relays is not accurately predictable-such relays may fail with contacts open or contacts closed.
On the other hand, when relay moving contact spacing gaps, source to source, of at least three millimeters have been achieved, the twenty millisecond minimum switching times have not been obtained.
Another problem commonly observed on known dual-input, auto-switching power supplies is relay contact chattering at the power input switching points, for example, when the voltage from the primary power input gradually drops to its dropout level or rises back to its voltage pull-in level. Such relay chattering is caused by the relay coil triggering circuit reacting in an analog manner; that is, not in a precise, digital manner and can cause voltage irregularities at the power output with possible malfunction of electrical or electronic equipment receiving power from the output.
For these and other reasons, a principal objective of the present invention is to provide a dual-input, auto-switching power supply having a switching speed no greater than about twenty milliseconds while maintaining a relay moving contact spacing gap of at least about three millimeters, source to source. Another important objective of the present invention is to provide a dual-input, auto-switching power supply that eliminates relay chattering at the switching points.